The push-pull method is one of the methods for deriving a tracking error signal. Referring to FIGS. 5a to 5c, the push-pull method uses change in distribution of energy in a beam spot which is caused by light diffracted and reflected by a pit P on a disc D when a laser beams is deflected from a track of the disc. When the laser beam is properly centralized on the track, the light is equally diffracted to the right and the left as shown in FIG. 5b. Thus energy is equally distributed. On the other hand, if the tracking is off-center as shown in FIGS. 5a and 5c, the reflected beams are asystemmetrically diffracted. By obtaining the difference between the distributions of energy, it can be determined the direction in which the beam is deflected from the track.
Referring to FIG. 6, a conventional track-following servo system using the push-pull method has a photodetector 4 for detecting the spot of the reflected beams. The photodetector 4 has two detecting areas 2 and 3 which are defined by a central boundary line 1 in the tangential direction of the disc. The reflected beam forms a shadow by a hatched area in FIG. 6 on each of the detecting areas 2 and 3. The areas of the shadows are applied to a differential amplifier 5 which applies a positive or negative tracking error signal to a track-following servo circuit 6. The circuit 6 operates an actuator 7 for positioning the optical pickup to render the difference of areas between the shadows on the detecting areas 2 and 3 zero.
If the track on the disc is properly followed, diffracting the beam as shown in FIG. 5b, the shadows formed on the detecting areas 2 and 3 have the same area so that the difference therebetween is zero. Hence the actuator 7 is not operated. If the beam is deflected to the left of the pit, thereby giving a diffraction shown in FIG. 5a, the shadow on the detecting area 2 is smaller than the shadow on the detecting area 3. To the contrary, if the beam is deflected to the right, so that the beam diffracts as shown in FIG. 5c, the shadow on the area 3 becomes smaller than the shadow on the area 2. Thus a difference is obtained by the differential amplifier 5, thereby applying a tracking error signal to the track-following servo circuit 6. As a result, the actuator 7 is operated.
In the push-pull method, when the axis of the laser beam is not vertical to the recording surface of the disc, or the objective is moved, or the axis of the laser beam is deflected, the spot of the reflected beam is deflected in the direction perpendicular to the center line 1. Consequently, the distribution of the energy of the spot received by the detecting areas 2 and 3 fluctuates, thereby causing the tracking error signal to have a DC offset. The track-following servo circuit 6 is operated in accordance with the erroneously offset tracking error signal so that the beam is further deflected from the track.
In order to reduce the DC offset, a part of the photodetector 4 is masked to render the size of the detecting areas 2 and 3 smaller than the spot as shown in FIG. 6. Elliptic masks may be further formed on the photodetector 4 as shown in FIG. 7.
However, since each mask must be designed to conform with the reflected beam, which is very small, it is difficult to provide an appropriate mask. Furthermore, the spot size of the light beam must be adjusted in accordance with the mask, which is also quite difficult.